The concept of injection impact compression moulding is described in WO02/258909 and is applicable in particular to the moulding of articles, such as food containers and cups, that have a large flow length to thickness ratio. In this moulding technique, a measured dose of a plastics material is introduced into a mould cavity prior to the mould parts being brought together fully. At the time of its injection, the dose of plastics material does not fill the cavity which still retains some empty spaces. The rapid closing of the mould cavity is then relied upon to compress the plastics material and make it flow into the thin walled sections and to fill the mould cavity completely.
This technique differs from injection compression moulding, as used in the manufacture of lenses and compact disks where the mould cavity is filled at the end of the injection step and compression is used only to take up the shrinkage volume as the article cools down. In impact moulding, compression is used to fill the mould cavity and force the molten plastics material to flow into gaps into which it could not be made to penetrate by the action of the injection screw.
WO02/058909 describes a mould for mounting between the platens of an injection moulding machine for injection compression moulding of a thin walled article. The mould comprises a female mould part mounted on the stationary platen of a moulding machine and a pressure plate which is mounted on the moving platen and carries a core for closing the mould. The core passes through, and is sealed relative to, a cylindrical bore in a rim closure ring, which need not be circular, arranged between the female mould half and the pressure plate. In use, as the pressure plate is advanced towards the stationary mould half, the rim closure ring is used to seal the mould cavity before the core reaches its end position. Thus, when the plastics material is injected into the mould cavity, it is fully sealed even though the core has yet to be fully advanced into the mould to reduce its volume to its smallest size.
When using such a mould, or one operating on similar principles, for impact moulding, it is essential to maintain the core accurately centred on and in parallel alignment with the cavity in the female mould part not only when the mould is fully closed but throughout the movement of the core while the injected plastics material is being compressed. When the moulded article has a very thin wall thickness, a misalignment of as little as 0.1 mm can be totally unacceptable. This is not just because of the variation in the wall thickness of the finished article but because, during compression, the plastics material flows along the path of least resistance and variation in wall thickness results in the plastics material flowing at different rates up the sides of the cavity and reaching the rim at different times.
The conventional approach for aligning the parts of a mould in an injection moulding machine is to provide conical projections on one part to engage in conical recesses in the other as the mould closes. This ensures that the mould parts are correctly aligned before injection commences. However, this approach does not satisfy the requirements of the impact moulding technique because injection and compression commence before the mould is fully closed, that is to say before the conical projections have fully engaged with the conical recesses. Furthermore, the guiding of the machine platens relative to one another by the tie bars or other platen guiding system of the injection moulding machine does not guarantee alignment of the mould parts to the required degree of accuracy.
The above problem is addressed in GB 2,410,000 which teaches a mould comprising a female mould part and a core part mounted in use on the platens of an injection moulding machine for movement towards and away from one another between an open and a closed position. At least one of the parts is adjustable relative to the associated platen to permit the centres of the two parts to be aligned with one another. Three or more flat guide fingers are provided on one of the mould parts and distributed about the core axis to be received in recesses of complementary shape in the other mould part as the mould parts approach the fully closed position. Each finger has two parallel sided locating sections that are spaced from one another along the length of the finger. The locating section nearer the free end of the finger is narrower than the other and the length of each locating section is at least equal to the final part of the stroke of the platens during which compression of the injected plastics material takes place.
While the latter proposal enables the mould parts to be moved into concentric alignment with one another, it does not guarantee that the axes of the mould parts are parallel to one another at all times during the compression part of the stroke of the platens.
To elaborate on this point, it is important in the impact moulding technique to minimise the trapping of air in the mould because compression of the plastics material by rapid closure of the mould heats the trapped air and causes burn marks in the finished article. WO02/058909 therefore teaches what is now termed a “shuffle” movement of the core. In essence, this involves closing the cavity completely before injection of the dose of molten plastics material commences and retracting the core to make room in the cavity for the plastics material. Once an accurately metered full dose of plastics material has been injected, the core is advanced a second time to compress the molten plastics material and force it to fill the entire mould cavity. A simple manner of achieving this shuffle is to support the core part so that it can move axially relative to the platen on which it is supported and to urge it away from that platen using a force sufficiently weak to be overcome by the pressure exerted by the injection screw.
In WO02/058909, it is assumed that the core part is supported on the platen in such a manner that it remains perfectly parallel to the face of the platen as it moves towards and away from it. However, the support bearings and the tie rods of the injection moulding machine are not able to guarantee correct alignment of the core plate to the degree of accuracy that it is required during impact moulding especially in view of the considerable weight of the core part of the mould and the closure plate that it carries.